Polarization mode dispersion (PMD) is a dispersion factor in optical fibers that can limit the bandwidth, or data rate, of fiber optics networks. As data transmission rates in optical fibers are exceeding ten gigabits/second (Gb/s) and fast approaching forty Gb/s and more, the PMD of the optical fiber indicates whether such transmission rates will be successful.
PMD is caused by the birefringence of the optical fiber. Birefringence is a property of the fiber in which the incoming light is split into two orthogonal propagation modes. Where each of these modes travels along the optical fiber at the same velocity, the transmitted data may be detected at the receiver without error. Sometimes, however, due to imperfections in the optical fiber (such as the fiber having an elliptical core rather than a circular one) the two modes may travel through the fiber at a different velocity, known as differential group delay dispersion (DGD). The DGD causes a broadening of the optical pulses at the other end of the fiber, such that a receiver is unable to reliably read the transmitted data.
To achieve a high transmission rate of an optical communications system, the data is transmitted with optical pulses that are shorter, relative to lower-rate transmission systems. The PMD characteristic of the optical fiber indicates whether such high-rate transmission will be successful. Thus, PMD testing is performed as part of a standard optical fiber installation or upgrade.
There exist several methods for performing PMD analysis: fixed analyzer, interferometry, and stokes parameter evaluation, to name a few. Most instruments for testing PMD in the field are either based on the interferometric method or the fixed analyzer method. While the data obtained by the interferometric method is in the time domain, the fixed analyzer method obtains data in the frequency domain. Both are standard methods recommended by ITU (International Telecommunication Union) and ANSI (American National Standards Institute), and the measurement results are equivalent using each method.
The fixed analyzer method (also called the wavelength scanning method) takes a measurement comparable to that obtained by an optical spectrum analyzer. A broadband polarized light source is launched at one end of the optical fiber and the spectrum representative of the light leaving the fiber at the other end is analyzed. Since the PMD of an optical fiber is proportional to the number of extrema found in the spectrum of the transmitted light signal, the PMD is obtained by counting the extrema of the spectrum representing the received light.
A simplified block diagram of a fixed analyzer method 50A is depicted in FIG. 1A, according to the prior art. An emitter 20 includes a broadband polarized light source 22, which sends light 24A into an optical fiber, or fiber under test (FUT) 30. At the other end of the FUT 30, the transmitted light 44A is detected by a receiver 40, which includes a fixed polarizer 42 and a scanning filter 48. The fixed polarizer 42 selects one linear polarization state 46 of the transmitted light 44A. The polarized light 46 is sent into the scanning filter 48, or tunable optical bandpass filter, which filters out all but a portion of the light 46, at a desired central wavelength. This operation is repeated while scanning the central wavelength over a predefined range. The extrema of this spectrum at each wavelength are counted to calculate the PMD of the optical fiber.
Another example of a fixed analyzer method 50B is depicted in FIG. 1B, according to the prior art. Instead of filtering the outgoing signal 44B from the optical fiber 30, the emitter 20 includes a polarized tunable light source 26, enabling the desired wavelength to be selected prior to transmission. Again, the extrema of the spectrum are counted at the receiver. The fixed analyzer methods 50A and 50B represent two of many possible implementations for performing PMD analysis.
The measured PMD value is proportional to the number of extrema found in the spectrogram. Referring to the example of FIG. 1A, the minimum measurable PMD value is given by the spectral width of the light source 22 and the wavelength tuning range of the filter 48. The maximum value is given by the linewidth of the tunable filter 48. Consequently, to obtain a wide PMD measurement range, the tunable filter 48 needs to have a wide tuning range and a narrow linewidth.
To obtain an accurate measurement using extrema counting, the FUT 30 needs to be stable (e.g., not moving). Controlling movement of the FUT is not always possible, however, such as when testing aerial cables during bad weather conditions. If the fiber is moving during the measurement, the output state of polarization can change, which leads to additional extrema in the recorded spectrum and a wrong PMD value. However, accurate measurements of moving optical fibers may be obtained when using a very fast tunable filter.
In the fixed analyzer method 50A (FIG. 1A), the scanning filter 48 in the receiver 40 is a tunable optical bandpass filter, used to make the wavelength scan. The filter 48 is typically set to start at the first desired wavelength. In a second step, the wavelength setting is increased to a next wavelength. The process is repeated until the end wavelength of the scan is selected. The optical bandpass filters are typically based on thin-film filters or diffraction gratings. Both types of filters need to be mechanically tuned by a motor, which limits the maximum achievable tuning speed. Such filters are not fast enough to obtain PMD measurements of fast-moving optical cables.
Thus, there is a continuing need for a fixed analyzer method for PMD testing that overcomes the shortcomings of the prior art, namely, a fixed analyzer having the ability to provide accurate PMD measurements of moving cables.